Update Python library handling

[olupton]

• Make libnrnpythonX.Y unconditionally minor-version-dependent, given that we do not use the Python limited API
  • Previously we used libnrnpythonX.Y on Windows but only libnrnpythonX on macOS and Linux
  • Avoid crashes due to ABI mismatch when updating to use PyConfig API
  • Update to use PyConfig API, avoiding warnings for outdated methods that are expected(?) to be removed in Python 3.12
  • Also allows us to inherit a more python-like configuration (e.g. for input encoding), reducing differences between python and nrniv -python
• Update logic for dynamically loading Python, as python or python3 is now more likely to not be a minor version that we compiled for.
• Overhaul CMake handling of Python to drop copy-pasted modules and hacky clearing of CACHE variables.
  • Now we search for Python in a subprocess and only transfer exactly the information that we need back.
• Add an extensive suite of "pyinit" tests that compare python, nrniv -python and many error cases.
  • NRN_PYTHON_EXTRA_FOR_TESTS option (list of valid Pythons that NEURON is not built against) allows explicit testing of error cases.
• Better support for virtual environments; things like -DPYTHON_EXECUTABLE=/path/to/venv/bin/python should just work
• nrn_find_python_module is now aware of multiple Python versions; ALL option checks if they all have a package
• documentation updates; dropped some duplicate HOC stuff in the Python section; added a new how-to-run-scripts page
• many nrnpy_* global variables are folded into a single neuron::python::methods struct, which holds pointers to functions that have to be built for specific versions of Python inside the libnrnpythonX.Y libraries; this makes the handling of such function pointers more consistent
  • not all of these variables have been updated; the work could be completed in a follow-up PR

[codecov]

Codecov Report

Merging #2317 (92e8ebf) into master (400e4c8) will increase coverage by 0.04%.
The diff coverage is 78.64%.

@@            Coverage Diff             @@
##           master    #2317      +/-   ##
==========================================
+ Coverage   58.79%   58.84%   +0.04%     
==========================================
  Files         625      626       +1     
  Lines      119921   119960      +39     
==========================================
+ Hits        70513    70587      +74     
+ Misses      49408    49373      -35     

Impacted Files	Coverage Δ
src/ivoc/classreg.cpp	100.00% <ø> (ø)
src/ivoc/datapath.cpp	65.38% <ø> (ø)
src/ivoc/graph.cpp	49.40% <ø> (ø)
src/ivoc/graphvec.cpp	69.69% <ø> (ø)
src/ivoc/grglyph.cpp	68.35% <ø> (ø)
src/ivoc/ivoc.cpp	81.94% <ø> (ø)
src/ivoc/ivocvect.cpp	85.08% <ø> (ø)
src/ivoc/mlinedit.cpp	3.75% <ø> (ø)
src/ivoc/ocbox.cpp	63.85% <ø> (ø)
src/ivoc/ocdeck.cpp	41.43% <ø> (ø)
... and 44 more

... and 4 files with indirect coverage changes

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[azure-pipelines]

✔️ 757d2f5 -> Azure artifacts URL

[azure-pipelines]

✔️ e063b3f -> Azure artifacts URL

[azure-pipelines]

✔️ 3926c5c -> Azure artifacts URL

[azure-pipelines]

✔️ c414dc6 -> Azure artifacts URL

[pramodk]

I didn't read the docs & cmake parts (but I see that Robert has at least looked at the docs).

The only thing that I looked into a bit more was at src/nrniv/nrnpy.cpp as historically this is the file where I/we were stumbling on. (In case @nrnhines wants to take a look at this PR, this could be one file to review maybe?)

With the improvements in CI & testing, this is really nice! 👌

[azure-pipelines]

✔️ 464b5e1 -> Azure artifacts URL

[azure-pipelines]

✔️ 92e8ebf -> Azure artifacts URL

[bbpbuildbot]

Logfiles from GitLab pipeline #117067 (:white_check_mark:) have been uploaded here!

Status and direct links:

• ✅ mac_m1_cmake_build: [cmake, ON, OFF, OFF, address]
• ✅ spack_setup
• ✅ build:nmodl
• ✅ build:neuron:mod2c:intel:shared
• ✅ build:neuron:nmodl:intel:legacy
• ✅ build:neuron:nmodl:intel:shared
• ✅ build:neuron:mod2c:nvhpc:acc:shared
• ✅ build:neuron:nmodl:nvhpc:acc:legacy
• ✅ build:neuron:nmodl:nvhpc:acc:shared
• ✅ build:neuron:nmodl:nvhpc:omp:legacy
• ✅ build:neuron:nmodl:nvhpc:omp
• ✅ test:neuron:mod2c:intel:shared
• ✅ test:neuron:nmodl:intel:legacy
• ✅ test:neuron:nmodl:intel:shared
• ✅ test:neuron:mod2c:nvhpc:acc:shared
• ✅ test:neuron:nmodl:nvhpc:acc:legacy
• ✅ test:neuron:nmodl:nvhpc:acc:shared
• ✅ test:neuron:nmodl:nvhpc:omp:legacy
• ✅ test:neuron:nmodl:nvhpc:omp